The hydration of alkali metal-containing silicate glass bodies in a steam-containing atmosphere to effect the diffusion of water into the glass structure and thereby endow the glass with thermoplastic properties is well-known to the art. An early disclosure of that practice can be found in U.S. Pat. No. 3,498,803 which describes the steam hydration of certain glass and glass-ceramic compositions to impart plastic or rubbery characteristics thereto. That patent cites anhydrous glass compositions operable in the process as consisting essentially, in mole percent on the oxide basis, of about 6-40% Na.sub.2 O and/or K.sub.2 O and 60-94% SiO.sub.2, the sum of those components constituting at least 85 mole percent of the total composition. As compatible metal oxides suitable as additives to modify the properties of the glasses, the patent notes the use of Al.sub.2 O.sub.3, BaO, B.sub.2 O.sub.3, MgO, PbO, P.sub.2 O.sub.5, and ZnO. CaO and Li.sub.2 O are mentioned as being preferably absent. The hydration process contemplated exposing the anhydrous glass to a gaseous environment containing at least 50% by weight H.sub.2 O at a pressure of at least one atmosphere and at a temperature customarily within the range of about 80.degree.-200.degree. C. The hydration treatment was undertaken for a sufficient length of time to cause the development of at least a surface layer on the glass bodies containing about 5-35% by weight water.
A companion disclosure, U.S. Pat. No. 3,498,802, is directed to the steam hydration of certain alkali metal-containing silicate glass powders, the penetration of water into the powders giving rise to thermoplastic properties and, in some instances, yielding products illustrating the characteristics of hydraulic cement. The anhydrous glass powders had compositions consisting essentially, in mole percent on the oxide basis, of about 6-20% Na.sub.2 O and/or K.sub.2 O and 80-94% SiO.sub.2, the sum of those ingredients constituting at least 90 mole percent of the composition. Again, Al.sub.2 O.sub.3, BaO, B.sub.2 O.sub.3, MgO, PbO, P.sub.2 O.sub.5, and ZnO are indicated as possible useful additions, whereas CaO and Li.sub.2 O are preferably to be avoided. The hydration process comprehended subjecting the glass powders to a gaseous atmosphere containing at least 50% by weight H.sub.2 O at a pressure of at least one atmosphere and a temperature normally between about 100.degree.-200.degree. C. Up to 30% by weight H.sub.2 O is diffused into the glass structure.
A number of the glass products produced via the disclosures of those patents exhibited such poor resistance to weathering and chemical durability that, except for protecting the surfaces thereof from the ambient environment, the articles rapidly lost their plastic or rubbery properties. Advantage was taken of that phenomenon in U.S. Pat. No. 3,811,853, which described the formation of alkali metal-containing silicate products that will, under certain conditions, self-degrade in the ambient environment. Such glasses have anhydrous compositions consisting essentially, in weight percent on the oxide basis, of 10-30% Na.sub.2 O and/or K.sub.2 O and 65-90% SiO.sub.2, the sum of those components constituting at least 80% of the total composition. The self-degradation phenomenon commences spontaneously after the weathering-resistant surface has been purposely penetrated or removed, thereby exposing the poorly resistant interior of the body to the ambient atmosphere.
In summary, the early prior art indicated that the hydration of glass, resulting in the pentration of H.sub.2 O into the structure of the glass, provides bodies which, demonstrating thermoplastic properties, can be molded or otherwise formed into useful shapes with relative ease. Unfortunately, however, hydration of the majority of the glass compositions in atmospheres saturated or near-saturated with steam yielded products of high water contents with less than practical chemical durability and resistance to weathering. Therefore, to enhance those characteristics, it has been found necessary to have a lower water content in the glass. Three processes have been developed for that purpose.
The first process is disclosed in U.S. Pat. No. 3,912,481. That method involved a two-step procedure wherein an alkali metal-containing silicate glass was initially hydrated in a saturated or near-saturated (at least 75% relative humidity) steam environment at elevated temperatures i.e., at temperatures above 100.degree. C., but below the softening point of the anhydrous glass, and pressures above atmospheric pressure and, thereafter, the glass was dehydrated in an atmosphere of lower relative humidity. The dehydration step is conducted in such a manner that the amount of water remaining in the glass can be carefully controlled to furnish the desired thermoplasticity to the glass, while also insuring satisfactory durability. Anhydrous glass compositions operable in that method consist essentially, in mole percent on the oxide basis, of 3-25% Na.sub.2 O and/or K.sub.2 O and 50-95% SiO.sub.2, the sum of those constituents composing at least 55 mole percent of the total composition. As useful optional additions, the patent notes the utility of up to 25% Al.sub.2 O.sub.3, up to 20% BaO, up to 25% B.sub.2 O.sub.3, up to 25% CaO, up to 35% MgO, up to 20% PbO, and up to 25% ZnO.
The second process is disclosed in U.S. application Ser. No. 445,454, filed Feb. 25, 1974 in the names of Pierson and Tarcza, abandoned in favor of continuation-in-part Ser. No. 822,877, filed Aug. 8, 1977. The method described therein is explicitly designed for hydrating fine-dimensioned articles, e.g., beads, granules, powders, and ribbon, of alkali metal-containing silicate glasses. The method comprehends a one-step process at high temperatures (higher than 225.degree. C. but below the softening point of the anhydrous glass) wherein the hydrating environment is of low relative humidity (less than 50%). The low steam content of the hydrating atmosphere results in a relatively slow rate of water penetration into the glass such that control of the water content taken up by the glass can be had. The process is useful with glasses having compositions like those operable in U.S. Pat. No. 3,912,481. However, since the water content of the steam environment is low, this method is drawn to fine-dimensioned bodies where the water introduced through the steam treatment can diffuse therethrough within a not unreasonably long space of time.
The third process is disclosed in U.S. Pat. No. 3,948,629. That method is founded in a one-step process wherein fine-dimensioned anhydrous glass bodies are hydrated in an acidic aqueous solution, i.e., an aqueous solution containing an acid or salt to yield a pH of less than 6, at temperatures between about 100.degree.-374.degree. C. and pressures in excess of 20 psig. By varying the glass composition, the composition of the hydrating solution, and the physical parameters of temperature and pressure, it is possible to closely control the amount of water taken into the glass structure. The process is operable with anhydrous glass compositions of the type reported in U.S. Pat. No. 3,912,481.
Each of the above three processes, while indeed permitting control of water content within the glass structure to be exercised, has certain drawbacks. For example, the two-step method described in U.S. Pat. No. 3,912,481 requires much care to be undertaken during the dehydration step to prevent foaming. Consequently, the dehydration step normally demands the presence of a nitrogen or other inert gas overpressure to avoid foaming. The single-step process of U.S. application Ser. No. 822,877 is quite slow and employs high temperatures, both of those factors being disadvantageous from a commercial point of view. Finally, care must be exercised in the solution hydration process of U.S. Pat. No. 3,948,629 to avoid attack upon and leaching of the glass.
The thermoplastic character of the hydrated glasses produced in accordance with the methods of those inventions has led to considerable research being undertaken to form shapes of various geometries therefrom. For example, articles of widely-varying configurations and dimensions have been compression molded therefrom. Whereas the glasses do, indeed, demonstrate a plastic character, when compared vis-a-vis with anhydrous glass, the mechanical strength thereof was only about equivalent to that of anhydrous glass and the impact resistance only slightly improved.
One application which has been the subject of extensive research has centered around the pressing of ophthalmic lenses from hydrosilicate glasses. The United States Food and Drug Administration has decreed [Federal Register 36(95):8939,1971] that to be used for such purpose a 2 mm thick section of glass must withstand the impact resulting from the fall of a stainless steel ball having a diameter of 5/8" from the height of 50". Hydrosilicate glasses produced in the conventional manner would not invariably pass that test successfully.